1. Field of Invention
The present invention relates to real-time in-situ monitoring of contaminants, such as alkali compounds, from a hot gas stream.
2. Background
The increasing use of dirty fuels for firing gas turbines for many applications requires the monitoring of gas contaminants that degrade the turbine performance by deposition, erosion and corrosion. Various contaminants can cause degradation, such as alkali compounds.
Alkali is the most common contaminant which causes severe degradation of turbine gas path components, and has a declared tolerance level of about 0.024 ppm for gases entering industrial turbines. This consideration arises because alkali salts, such as sulfates, cause sever corrosion of turbine parts when condensed on the gas path components. As the reserves of clean fuels are decreasing, turbine systems are operating with increasingly dirtier fuels which enhances the likelihood of such corrosion.
Coal-fired systems are formulated where pressurized coal combustion gases pass through a gas turbine. Coal combustion gases are reported to have alkali concentrations of from 0.05 ppm to several ppm; see S. Lee, et al., "Measurement of Alkali Vapor Concentration in PFBC Flue Gas," Proceedings of 1989 International Conference on Fluidized Bed Combustion, ASME, pp 977-985, May 1989; hereinafter Lee (1989). Various physical and chemical systems can potentially remove such excess contaminants from pre-turbine hot gases; however, the key is still the appropriate detection of contaminant amounts.
Another dirty hot gas environment is that from the combustion of municipal solid wastes where heavy elements, usually in the form of salts, are potentially a concern. Such salts cause severe corrosion of water-wall tubes, superheater tubes and other components.
Current alkali monitors are based upon several procedures. Extractive condensation employs a filter to remove particulate matter followed by a cold trap and water bubblers to condense alkali compounds; see H. Spacil, et al., "Volatization/Condensation of Alkali Salts in a Pressured Fluidized Bed Coal Combustor/Gas Turbine Combined Cycle," J. Electrochem. Soc., 129, pp 2119-2126, September 1982.
Sorber beds employ activated bauxite or diatomaceous earth to remove alkali from a slip stream followed by water leaching and conventional wet analysis of the sorber bed materials; see Lee (1989).
Vapor monitors employ flame emission spectroscopy applied to a slip stream; see W. Hass, et al., "Fiber Optic Alkali Meter Sampling," Proceeding of the Seventh Annual Coal-Fueled Heat Engines and Gas Stream Cleanup Systems Review Meeting, Editors H. Webb, et al., DOE/METC-90/6110, pp 391-402, Mar. 1990.
Much variability is shown by comparisons between these above methods, and potentially sampling line losses may represent a plausible source for these discrepancies. None of these methods employ real-time in-situ measurement which negates sampling line problems.